1. Field of the Invention
The present invention relates to a piezoelectric transducer and a liquid droplet ejection device that employs the piezoelectric transducer.
2. Description of Related Art
There has been proposed a print head of a drop-on-demand type. This print head employs a piezoelectric type liquid droplet ejection device that prints letters, characters, and images on a sheet of paper in a dot-matrix form. More specifically, the piezoelectric type liquid droplet ejection device includes a number of ink ejection units, which are arranged close to one another. Each ink ejection unit includes a piezoelectric transducer and a liquid chamber that is filled with ink. By changing the size of the piezoelectric transducer, it is possible to change the volume of the liquid chamber. When the volume decreases, ink is ejected out the liquid chamber via a corresponding nozzle. When the volume increases, ink is supplied into the liquid chamber from an ink supply source. By controlling ink ejection units at selected positions, it is possible to produce desired letters, desired characters, and desired images.
There has been a piezoelectric type liquid droplet ejection device, in which a piezoelectric plate is provided over a plurality of liquid chambers. In this type of device, the piezoelectric plate is deformed locally at a desired position that corresponds to a selected liquid chamber. In order to deform piezoelectric material, there have been proposed two types of modes: a direct mode, and a shear mode.
The direct mode is described in the U.S. Pat. No. 5,402,159. According to the direct mode described in this publication, a plurality of piezoelectric ceramic layers are stacked one on another. Positive and negative electrodes are alternately disposed in the laminated piezoelectric ceramic layers so that each electrode is sandwiched between two adjacent piezoelectric ceramic layers. Each piezoelectric ceramic layer, located between positive and negative electrodes, is polarized in a direction along which the positive and negative electrodes oppose with each other. When a voltage is applied between the positive and negative electrodes, the piezoelectric ceramic layer expands along the polarized direction due to an electric field that extends in the same direction with the polarized direction. As a result, the volume of the corresponding liquid chamber is changed, and ink is applied with a pressure and is ejected.
The shear mode is described in the U.S. Pat. No. 5,266,964. Also according to the shear mode described in this publication, a plurality of piezoelectric ceramic layers are stacked one an another. A group of positive electrodes is provided in the laminated piezoelectric ceramic layers so that each positive electrode is sandwiched between two adjacent piezoelectric ceramic layers. A group of negative electrodes is provided in the laminated piezoelectric ceramic layers at a location that is distant from the position where the positive electrode group is provided in a direction perpendicular to the direction, in which the piezoelectric ceramic layers are laminated. Also in the negative electrode group, each negative electrode is sandwiched between two adjacent piezoelectric ceramic layers. The part of the piezoelectric-ceramic layer lamination, between the positive electrode group and the negative electrode group, is polarized in a direction perpendicular to the direction, in which the positive electrode group and the negative electrode group oppose with each other. When a voltage is applied between the positive and negative electrode groups, the part of the piezoelectric ceramic layer lamination, between those electrodes, is applied with an electric field that is perpendicular to the polarized direction. As a result, the piezoelectric ceramic layer lamination is deformed in a shear-mode, that is, in a parallelogram shape. As a result, the volume of a corresponding liquid chamber is changed, and ink is applied with a pressure and is ejected.
In the direct mode, however, it is necessary to stack a great number of ceramic layers and a great number of electrodes one on another in order to attain a desired large amount of deformation. In the shear mode, each positive (negative) electrode is oriented so that its surface does not oppose to the corresponding negative (positive) electrode. Each positive (negative) electrode faces in a direction perpendicular to the direction, in which the electrode opposes with the negative (positive) electrode. It is therefore necessary to provide a large number of electrodes in each group of electrodes 50 that it appears that each electrode group has a large amount of area opposed with a corresponding opposite-polarity electrode group.
In this way, it is necessary to employ a great number of steps to manufacture the liquid droplet ejection device of each mode. More specifically, in order to manufacture the liquid droplet ejection device of each mode, a great number of ceramic green sheets are first prepared A plurality of electrodes are formed on each green sheet, by providing electrode material on the green sheet by screen printing or vapor deposition. On each green sheet, the plurality of electrodes are located at positions corresponding to a plurality of liquid chambers. The green sheets thus formed with the electrodes are then stacked one on another. At this process, it is necessary to stack the green sheets so that the electrodes on the green sheets will be located one on another accurately.
In view of the above-described drawbacks, it is an objective of the present invention to provide an improved piezoelectric transducer that can obtain a desired large amount of deformation even with a small number of electrodes and to provide an improved liquid droplet ejection device that employs the improved piezoelectric transducer
In order to attain the above and other objects, the present invention provides a piezoelectric transducer, comprising: a piezoelectric plate, which is made of piezoelectric material and which has a pair of opposite surfaces, the piezoelectric plate having at least one actuating portion desired to be deformed and at least two non-actuating portions, each actuating portion being located as being interposed between corresponding two non-actuating portions, each actuating portion having a center, the piezoelectric plate being polarized in a pair of polarized directions, which are slanted with respect to both of a surface direction and a thickness direction and which are symmetrical with respect to the center of each actuating portion, the surface direction being defined along the opposite surfaces of the piezoelectric plate, the thickness direction being defined along a thickness of the piezoelectric plate and substantially perpendicular to the surface direction; and a pair of driving electrodes, each of which is provided on a corresponding surface of the piezoelectric plate, the pair of driving electrodes being for applying an electric field that extends substantially perpendicularly to the polarized directions, thereby causing the actuating portion to be deformed in a direction substantially perpendicular to the surface direction, that is, in a shear-mode.
It is possible to polarize the piezoelectric plate in the direction slanted with respect to both of the surface direction and the thickness direction, by providing a pair of polarizing electrodes at positions so that the pair of polarizing electrodes will oppose with each other along a direction slanted with respect to the surface direction. In such a cases an imaginary line connecting between the pair of polarizing electrodes extends along the direction slanted with respect to the surface direction.
It is noted that the pair of polarizing electrodes may be provided outside the piezoelectric plate, or inside the piezoelectric plate but at locations near to the opposite surfaces of the piezoelectric plate.
It is possible to effectively deform the piezoelectric plate, by providing a pair of driving electrodes on the opposite surfaces of the piezoelectric plate so that they oppose with each other and so that they apply an electric field substantially perpendicularly to the slanted polarized directions. In this way, it is possible to effectively deform the piezoelectric plate even with using a small number of electrodes.
It is possible to deform the piezoelectric plate in a direction substantially perpendicular to the surface direction, by locating the polarizing electrodes symmetrically with respect to the center of each actuating portion and by locating the driving electrodes symmetrically with respect to the center of each actuating portion.
The piezoelectric plate may be provided with no polarizing electrodes. However, it may be possible to provide the piezoelectric plate with a pair of polarizing electrodes. For example, the pair of polarizing electrodes may be provided in the interior of the piezoelectric plate as internal polarizing electrodes.
According to another aspect, the present invention provides a piezoelectric transducer, comprising: a piezoelectric plate which is made of piezoelectric material and which has a pair of opposite surfaces, the pair of opposite surfaces extending in a predetermined surface direction and being opposed to each other along a predetermined thickness direction, the predetermined thickness direction being substantially perpendicular to the predetermined surface direction; a first electrode group and a second electrode group provided to the piezoelectric plate, the first electrode group and the second electrode group being distant from each other in the thickness direction, the first electrode group including a plurality of first electrodes arranged in the surface direction as being separated from one another, and the second electrode group including a plurality of second electrodes arranged in the surface direction as being separated from one another, the plurality of first and second electrodes including; at least one polarizing combination of first and second electrodes, between which a polarizing electric field is to be applied to polarize the piezoelectric plate; and at least one driving combination of first and second electrodes, between which a driving electric field is to be applied to actuate the piezoelectric plate, the driving combination of first and second electrodes being different from the polarizing combination of first and second electrodes, an imaginary line connecting between the driving combination of first and second electrodes substantially intersecting with an imaginary line connecting between the polarizing combination of first and second electrodes, thereby allowing the piezoelectric plate to be deformed in a shear mode fashion, that is, substantially perpendicularly to the surface direction, upon driven by the driving combination of first and second electrodes.
According to another aspect, the present invention provides a liquid droplet ejection device, comprising: a piezoelectric plate, which is made of piezoelectric material and which has a pair of opposite surfaces, the piezoelectric plate having at least one actuating portion desired to be deformed, the pair of opposite surfaces extending in a predetermined surface direction and being opposed to each other along a predetermined thickness direction, the predetermined thickness direction being substantially perpendicular to the predetermined surface direction; and a wall having at least two partition walls that define at least one liquid chamber therebetween, the liquid chamber being filled with liquid, the wall being connected to one of the pair of opposite surfaces of the piezoelectric plate so that each actuating portion in the piezoelectric plate is located at a position corresponding to a corresponding liquid chamber, the center of the actuating portion corresponding to the center of the liquid chamber, the piezoelectric plate being polarized in a pair of polarized directions at a pair of polarized portions in each actuating portion, the pair of polarized portions being defined as a pair of regions between a position corresponding to the center of the liquid chamber and a position corresponding to the two partition walls that sandwich the liquid chamber therebetween, the polarized directions being symmetrical with each other with respect to the center of the liquid chamber and slanted with respect to both of the thickness direction and the surface direction; and a pair of driving electrodes, each of which is provided on a corresponding surface of the piezoelectric plate, the pair of driving electrodes being for applying an electric field that extends substantially perpendicularly to the polarized directions, thereby causing the actuating portion to be deformed in a direction substantially perpendicular to the surface direction, that is, in a shear-mode fashion, to thereby change the volume of the liquid chamber and allow the liquid to be ejected from the liquid chamber.
According to another aspect, the present invention provides a liquid droplet ejection device, comprising: a piezoelectric plate which is made of piezoelectric material and which has a pair of opposite surfaces, the pair of opposite surfaces extending in a predetermined surface direction and being opposed to each other along a predetermined thickness direction, the predetermined thickness direction being substantially perpendicular to the predetermined surface direction; a liquid chamber unit defining a plurality of liquid chambers, the liquid chamber unit being connected to one of the pair of opposite surfaces of the piezoelectric plate, the piezoelectric plate being provided over the plurality of liquid chambers; a first electrode group and a second electrode group provided to the piezoelectric plate, the first electrode group and the second electrode group being distant from each other in the thickness direction, the first electrode group including a plurality of first electrodes arranged in the surface direction as being separated from one another, the second electrode group including a plurality of second electrodes arranged in the surface direction as being separated from one another; and an energizing unit applying a polarizing electric field between at least one polarizing combination of first and second electrodes, and applying a driving electric field between at least one driving combination of first and second electrodes, the driving combination of first and second electrodes being different from the polarizing combination of first and second electrodes, an imaginary line connecting between the driving combination of first and second electrodes substantially intersecting with an imaginary line connecting between the polarizing combination of first and second electrodes, whereby the energizing unit allows the piezoelectric plate to be deformed in a shear mode fashion, that is, substantially perpendicularly to the surface direction, when applying the driving electric field between the driving combination of first and second electrodes, thereby allowing the volume of the liquid chamber to be changed and allowing the liquid chamber to eject a liquid droplet therefrom.